analysis: Data analysis packages.

Description

The `st4gem.analysis' package contains eight packages of tasks for general image and spectral analysis, image restoration, statistics, and access to the Guide Star Catalog. These tasks are not necessarily specific to HST data, but they do address some of the most common anaylsis needs at Space Telescope Science Institute. In addition, these tasks are designed to make use of both IRAF images, and ST4GEM binary tables for task I/O. A quick summary of the available packages is given in Table 1 below; a more detailed summary can be found in the following sections.


            Table 1.  HST Analysis Packages
+-----------------------------------------------------------+
| Package    | Description                                  |
+-----------------------------------------------------------+
| dither     | Combine images using the "drizzle" algorithm |
| fitting    | General image and spectrum feature fitting   |
| fourier    | Utilities for Fourier analysis               |
| gasp       | Guide-Star Astrometric Support Package       |
| isophote   | Elliptical isophote fitting                  |
| nebular    | Tasks for analyzing nebular emission lines   |
| restore    | Image deconvolution & restoration tasks      |
| slitless   | Extracting and simulating slitless data      |
| statistics | Statistics for censored data (ASURV V1.2)    |
+-----------------------------------------------------------+

DITHERED IMAGE COMBINATION

The `dither' package contains tasks which can be used to combine dithered images using the "drizzle" algorithm.

Two basic procedures are available, depending on the existence of only linear shifts between the images to be combined, or linear shifts plus a (small) rotation angle. Both procedures rely on cross-correlation to derive the relative shifts and rotation angles.

The `drizzle' task can be used to process any image format supported by IRAF, the remaining tasks are specific for WFPC data. The `drizzle' task must be supplied with the linear shifts in X and Y and the rotation angle in between images to be combined. It also supports rescaling the pixel grid and corrections for geometric distortions, but only coefficients appropriate for WFPC-II data are currently available.

GENERAL FEATURE FITTING

There are two packages with tasks that are useful for fitting features in images, and both are layered on the IRAF 'icfit' utilities. That is, the majority of the 'cursor' commands that are defined for 'icfit' will function for these fitting tasks as well. Both packages use the 'tables' external package for storing and accessing the fitted parameters.

The `isophote' package is intended for fitting galaxy brightness profiles in 2-D images with elliptical isophotes. The more general `fitting' package can fit elliptical Gaussians to two-dimensional images. It can also fit one-dimensional Gaussians, Planck or power-law functions, and composite profiles to spectra or image sections. The `fitting' tasks can optionally set or solve for any of the function coefficients. The functions are fit using a downhill simplex (or "amoeba") method, and the coefficient errors can be estimated using a "bootstrap" technique.

FOURIER ANALYSIS

The `fourier' package contains tasks for forward and inverse Fourier transforms, as well as related tasks for computing power spectra, cross-correlations, convolutions, etc. Other auxiliary tasks can find the prime factors of a number, perform complex image arithmetic, or taper image edges. At present, these tasks operate only on images, not on tables or lists.

GUIDE STAR ANALYSIS

The `gasp' package contains utilities for searching the HST Guide Star Catalog index (on CD-ROM), building ST4GEM tables with extracted portions, and producing charts of guide stars. It is also possible to use guide stars to derive a plate solution for an image, determine the celestial coordinates of astronomical objects in that image, and write FITS-style World Coordinate System descriptors into an image header based up the plate solution.

NEBULAR ANALYSIS

Tasks in the `nebular' package can be used to derive various physical quantities from the emission line fluxes of a low-density (nebular) gas. These quantities include the electron temperature (T_e) and density (N_e) from various diagnostic line flux ratios, and level populations, critical densities, line emissivities and abundances for most common ions.

IMAGE RESTORATION

Ever since spherical aberration was discovered in the HST primary mirror, interest has grown in the optical-UV community in image deconvolution and restoration techniques. The `restore' package contains several tasks for performing restorations with various methods, including the standard "Weiner" filter, the "Lucy" method, a "Sigma-clean", and a public-domain Maximum-Entropy algorithm. All of these tasks have been tailored to be useful for restoring HST data, and some have been substantially enhanced for greater performance, features, and reliability. Each task is more or less appropriate for a given type of data, depending upon the scientific goals, computing resources, and patience of the user.

SLITLESS SPECTROSCOPY DATA

Details still to be written......

STATISTICS OF CENSORED DATA

Very often in astronomical analysis one is faced with the job of performing some statistical analysis on a dataset that contains upper or lower detection limits. Often there is important information in, e.g., the failure to detect an object in a survey, but the simple and familiar statistical methods require that these data be excluded from the analysis. The proper techniques for dealing with these so-called "censored" data are collectively referred to as "survival analysis". The `statistics' package is a collection of tasks from the ASURV V1.2 package, which was developed by Isobe, LaValley, and Feigelson at Penn State Univ. These tasks all make use of binary or ASCII tables for input.

OTHER ANALYSIS TASKS

Some analysis tasks have been written outside the ST4GEM group, but are potentially very useful for the HST community. We make these tasks available in the `contrib' package, although the responsibility for the accuracy and utility of these tasks rests with the authors.

analysis: Data analysis packages.

Description

See also